1. Field of the Invention
The present invention generality relates to radiative energy and apparatuses thereof, in particular it relates to a radiation generating tube that generates radiation rays by irradiating a target with electrons and that can be applied to radiography. The present invention also relates to a radiation generating device using the radiation generating tube, and to a radiation imaging apparatus using the radiation generating device.
2. Description of Related Art
A radiation generating device used as a radiation source generates radiation rays by emitting electrons from an electron source, and causing the electrons to collide with a target electrode. The radiation source and the target are typically arranged within a radiation generating tube, which is kept in a vacuum. The radiation is generated, by bringing the electrons into collision with the target, which is made of a metal material having a large atomic number, such as tungsten.
To efficiently generate electrons from the electron source and promote an increase in life of the radiation generating device, it is necessary to keep the inside of the radiation generating tube in vacuum for a long period. Therefore, appropriate sealing of the vacuum environment is necessary. Also, in the radiation generating device, since radiation rays generated from the target are radiated in all directions, unnecessary radiation rays other than radiation rays necessary for imaging are typically blocked by providing a rear shield body. Japanese Patent Application Laid-Open No. 2012-124098 discloses a radiation generating device using a transparent target. The technique disclosed in Japanese Patent Application Laid-Open No. 2012-124098 keeps the inside of a radiation generating tube in a vacuum by brazing the periphery of a transparent substrate having a target layer to a shield body of the radiation generating tube.
Therefore, a known method of keeping the radiation generating device in vacuum and blocking unnecessary radiation rays may be a method of sealing a radiation generating tube in vacuum by brazing a target to the above-described rear shield body. A brazing material used for joining is a low-melting-point material having a melting point lower than the melting points of both the target material and the rear shield body. Owing to this, the brazing material that joins the peripheral edge portion of the target with the shield body may be softened and molten by heat generated when the radiation generating tube is operated, and the brazing material may flow to a target-layer formation region or an electron passage. In this case, if an electron beam is emitted onto the flowing brazing material, a radiation ray with a radiation quality different than the radiation quality caused by the target material alone may be radiated forward, and consequently the radiation quality caused by the target material may be decreased.